In a fuel pump known to the art, a substantially disc-shaped impeller is rotated within a casing, whereby fuel is drawn from outside the casing to within the casing, the pressure of the fuel is increased within the casing, and the pressurized fuel is discharged to the exterior of the casing. An example of this type of fuel pump is shown in FIGS. 10 to 14. FIG. 10 is a cross-sectional view of a conventional fuel pump, FIG. 11 is a figure, viewed from an inner side of the casing, of an impeller 16 in a fitted state within a pump cover 9, FIG. 12 is a figure, viewed from the inner side of the casing, of pump cover 9, FIG. 13 is a figure, viewed from the inner side of the casing, of a pump body 15, and FIG. 14 is a figure schematically showing the flow of fuel.
As shown in FIG. 10, the fuel pump comprises a pump portion 1 and a motor portion 2 for driving pump portion 1. Pump portion 1 and motor portion 2 are unified by a housing 4.
Pump portion 1 comprises a pump cover 9, a pump body 15, and a substantially disc-shaped impeller 16, etc. Pump cover 9 and pump body 15, by being fitted together, form a casing 17 wherein impeller 16 is housed.
As shown in FIG. 11, impeller 16 is substantially disc shaped, and a group of concavities 16a is formed in an area thereof inwards from an impeller outer circumference face 16p by a specified d distance, the group of concavities 16a being formed along a circumference direction thereof. Adjacent concavities 16a are separated by partition walls 16d that extend in a radial direction. Concavities 16a and partition walls 16d form the group of concavities 16a that are repeated in a circumference direction. The group of concavities 16a is formed in both upper and lower faces of impeller 16, and base portions of each of the upper and lower concavities 16a communicate via a through-hole 16c (see FIG. 14).
As shown in FIGS. 10 and 12, a groove 21 is formed in a lower face of pump cover 9 in an area opposite the group of concavities 16a in the upper face of impeller 16. Groove 21 extends continuously in the direction of rotation of impeller 16 from an upper flow end 21a to a lower flow end 21c. A discharge hole 24 is formed in pump cover 9, discharge hole 24 extending from lower flow end 21c of groove 21 to an upper face of pump cover 9. Discharge hole 24 passes through from the interior of casing 17 to the exterior of casing 17 (an inner space 2a of motor portion 2).
As shown in FIG. 11, an inner circumference face 9c of a circumference wall 9b of pump cover 9 faces impeller outer circumference face 16p with a minute clearance C2 being formed therebetween. Inner circumference face 9c extends along almost the entire circumference of pump cover 9 (the region shown by the angle A shown in FIG. 11 being excepted therefrom). Inner circumference face 9c protrudes outwards in the radial direction at the region shown by the angle A in the vicinity of discharge hole 24, thereby ensuring a large clearance C1 between inner circumference face 9c and impeller outer circumference face 16p. 
As shown in FIGS. 11 and 12, groove 21, in the vicinity of lower flow end 21c thereof, extends in a tangential direction in a straight line to the radial outer side (see 21b), and discharge hole 24 protrudes further outwards than the group of concavities 16a of impeller 16. Discharge hole 24 also protrudes even further outwards than impeller outer circumference face 16p. 
As shown in FIGS. 10 and 13, a groove 20 is formed in an upper face of pump body 15 in an area opposite the group of concavities 16a in the lower face of impeller 16. Groove 20 extends continuously along the direction of rotation of impeller 16 (in FIGS. 12 and 13 the figures are viewed from a reverse direction and consequently the direction of rotation of the impeller is shown facing the reverse direction) from an upper flow end 20a to a lower flow end 20b. An intake hole 22 is formed in pump body 15, intake hole 22 extending from upper flow end 20a of groove 20 to a lower face of pump body 15. Intake hole 22 passes through from the interior to the exterior of casing 17.
Groove 21 extending in the circumference direction of pump cover 9, and groove 20 extending in the circumference direction of pump body 15, extend along the direction of rotation of impeller 16, and extend from intake hole 22 to discharge hole 24. When impeller 16 rotates, the fuel is drawn into casing 17 from intake hole 22, flows from intake hole 22 along grooves 20 and 21 towards discharge hole 24, the pressure of the fuel rising meanwhile, and then the pressurized fuel is delivered from discharge hole 24 to motor portion 2.
Discharge hole 24 communicates with a clearance 26 between impeller outer circumference face 16p and inner circumference face 9c of pump cover 9 (see FIGS. 11 and 14). As shown in FIG. 14, the fuel that has been pressurized by impeller 16 within groove 20 flows into discharge hole 24 via clearance 26 at the outer side of impeller outer circumference face 16p. 